A cost efficient solution for improving the performance of Long Term Evolution (LTE) and LTE-Advanced (LTE-A) telecommunication networks can be the utilization of relay nodes (RN), which allows installations without having terrestrial broad-band access or the need to install a micro wave link. In a relay enhance network there are basically three different types of connections: (A) A first type is the connection between a base station (BS), which in Long Term Evolution (LTE) technology is called an enhanced NodeB (eNB), and a RN. The RN serving BS is also called a donor BS. The respective cell is called a donor cell. (B) A second type is the connection between a BS a User Equipment (UE). (C) A third type is the connection between a RN and a UE.
The connection between a BS and a RN can be inband, in which case the BS-to-RN link shares the same frequency band with the RN-to-UE links within the donor cell. Alternatively, the connection can be outband, in which case the BS-to-RN link does not operate in the same frequency band as direct BS-to-UE links within the donor cell of the RN.
For inband relaying, the BS-to-RN link operates in the same frequency spectrum as the RN-to-UE link. Thereby, a transmitter of a RN may cause interference to its own receiver. Therefore, simultaneous BS-to-RN and RN-to-UE transmissions (full duplex transmission) on the same frequency band may not be feasible. This holds in particular in the case that traditional antenna and signal processing technology are used.
For outband relaying a similar problem as for inband relaying occurs in the case the BS-to-RN link and RN-to-UE link use adjacent frequency channels. The self interference between the RN-to-BS link and the RN-to-UE link still prevents the RN to perform full duplex transmission when the self isolation between the RN transmitter and the RN transceiver for adjacent channels is not large enough.
To avoid RN self interference due to a limited or a poor self isolation, a half duplex transmission solution is defined in 3GPP for LTE RNs. The RN does not transmit to UEs when it is supposed to receive data from its donor BS. This means that “time-gaps” are created in the RN-to-UE transmission. Compared to the full duplex transmission scheme, the transmission time of the half duplex transmission scheme is shorter for both the BS-to-RN link, which in the following is also called a relay link, and the RN-to-UE link, which in the following is also called access link. Therefore, the overall transmission efficiency is reduced and the transmission delay is increasing.
In order to realize a good self isolation of a RN different measures are known. (A) One effective measure is to use spatial multiplexing between the RN's relay antenna and the RN's access antenna. This means that different signal transmission directions between the RN-to-BS link and the UE-to-RN link are used. (B) A further known measure is to establish a big spatial distance between the transmitting antenna and the receiving antenna of a RN. In uplink (UL) radio transmissions the receiving antenna is used for receiving radio signals from at least one UE and the transmitting antenna is used for transmitting radio signals to the donor BS. Accordingly, in downlink (DL) radio transmissions the receiving antenna is used for receiving radio signals from the donor BS and the transmitting antenna is used for transmitting radio signals to at least one UE being connected by the RN. In case a sufficient radio self isolation is established and the RN supports full duplex transmission in hardware and software, the RN can be operated with full duplex transmission.
However, real radio telecommunication network environments are typically very complex and in some RN deployment scenarios it is hard to implement a good self isolation between the relay link and the access link for instance due to a strong signal reflection. The signal reflection may break the signal isolation generated for instance by the directional character of the antenna(s) of a RN. In this respect a directional character of a RN antenna is given if the signal transmission and reception pattern is limited to wanted spatial direction and the signal strength of Tx/Rx in unwanted direction will be attenuated.
It can be easily understood that an unwanted signal reflection will change the signal's transmission direction and a part of the unwanted signal will arrive to the RN receive antenna through the reflection. If the reflection signal's strength is big enough, the respective interference signal can even block the RN receiver. In this case even an advanced digital signal processing in order to suppress the interference cannot work.
For a RN, the strength of self interference being generated by Tx signal reflection to Rx side depends on (a) the location and the directional property of a radio reflector, (b) the directions of the RN transmit antenna and the RN receive antenna, and (c) the strength of the transmitted radio signal.
Moreover, the self interference strength of a RN may also depend on which UE the RN is serving. This may be the case when a directional antenna or beam forming is used for the RN-to-UE link and a radio reflector is present. Then, the radio signal reflection will depend on both the reflector's location and the UE's location which is pointed by the directional antenna radiation pattern. Therefore, the RN may serve (a) some UEs which are located in places that cause no or only little reflection and (b) other UEs that cause strong reflection. In case that the RN transmits to the UEs in the direction with no or only little reflection (case (a)), then good isolation between the RN-to-BS link and the RN-to-UE link is guaranteed and the RN may be operated with full duplex transmission. On the contrary, when the RN transmits to the UE in the direction with strong reflection (case (b)), then strong RN self interference might occur and the RN cannot be operated with full duplex transmission.
The above described situation will become even more complex if the reflector is mobile such as a bus passing through the coverage area of the RN. Therefore, the above described discrimination between UE's allowing for full duplex transmission and other UE's allowing only for half duplex transmission will even become time dependent.
In addition to RN self interference, a RN operating in full duplex transmission may also cause interference between the BS when transmitting to the RN and the RN transmitting to the UEs in DL direction. In case a RN operates in half duplex transmission mode, the BS may transmit to a UE which is connected to the BS while the RN transmits to a UE served by it. To avoid the interference of a transmitting BS to the UEs served by a RN, the BS may stop transmission in the RN direction or at least reduce the transmission power in RN direction. In a RN full duplex transmission mode, the BS transmits to the RN during the time that the RN transmits to the UE. This UE may suffer from interference originating from the TX signal of the BS. Thereby, the strength of this interference typically strongly depends on the current location of the UE.
Furthermore, in case of a RN operating in full duplex transmission mode there might occur interference between the RN-to-BS link (also called backhaul link) and the UE-to-RN link (access link) when the RN transmits in uplink on the backhaul link using the same radio resources which are also used for the uplink transmission by its connected UEs. This type of interference can also be called access-to-backhaul interference.
In summary, in order to enable a RN for performing full duplex transmission, which of cause improves the performance of the respective relay enhanced telecommunication network, at least the four following conditions should have been satisfied:
1. Of course the RN must have the capability for a full duplex transmission. Apart from having at least two sets of transceivers this might require in particular a good antenna for isolation and advanced interference cancellation algorithm.
2. The self interference strength between the RN-to-BS link (relay link) and the RN-to-UE link (access link) has to be below a predetermined RN work point. This work point may depend on various parameters such as for instance (a) the link rate, (b) the modulation and coding schemes used for TX and RX, (c) a factor of self interference suppression caused by advanced signal processing methods, (d) the self interference determined by the RN's position and/or (e) the position of a radio reflector and the position of the served UEs.
3. At least some UEs being served by or connected with the RN must not suffer from interference caused by the donor BS. If all the UEs suffer the interference from the donor BS, it is meaningless to enable the RN performing simultaneously TX and RX.
4. The UEs connected to RN do not produce too much interference to the RN-to-BS (i.e. uplink backhaul) transmission.
There may be a need for improving the performance of a relay enhanced radio telecommunication network.